U.S. Pat. No. 3,444,007 describes a process for forming a complex oxide corrosion-resistant coating on a zinc or zinc alloy surface comprising treating the surface with an aqueous alkaline solution having a pH of at least 11 and which contains a metal of a group of the periodic system other than group 1 and that to attain desired solubility is complexed with a complexing, chelating or sequestering agent and is thus in solution. Hexavalent chromium interferes with the coating and so must be generally absent, but it is desirable to give the coating a subsequent rinse in dilute aqueous chromic acid.
It is stated that somewhat better results are obtained with compositions which contain at least two complexed metal ions. The combinations of complexed metals that are exemplified are separate combinations of ferric iron with either cobalt, magnesium, cadmium, tin, titanium, antimony, bismuth, silver, molybdenum, tungsten or manganese and separate combinations of cobalt with either arsenic or cerium. Other complexed metals that are exemplified for individual use are ferrous iron and vanadium while aluminum, chromium and nickel are mentioned as metals that can be used in the process but are not exemplified.
A process that has been used successfully involves the treatment of continuous strip galvanized steel at, for instance, above 50.degree. C. using an alkaline solution of complexed cobalt and ferric iron, followed by a chromate rinse.
It would be desirable to improve the corrosion resistance and/or adhesion of the coating and it would be desirable to be able to obtain a good coating either using less concentrated solutions or lower coating temperatures or both.